1. Field of the Invention
This invention relates to a control system for an automatically guided vehicle (AGV) and more specifically a non-contact collision avoidance system for an AGV for preventing collisions between the vehicle and obstacles in the path of the vehicle during various vehicle maneuvers, including relatively sharp turns.
2. Description of the Prior Art
Automatically guided unmanned vehicles are generally known in the art. Examples of such vehicles are disclosed in the following U.S. Pat. Nos. 4,328,545; 4,345,662; 3,379,497; 4,623,032; 4,602,334; 4,627,511; 3,039,554; 3,610,363; 3,933,099; 4,003,445; 4,151,526; 4,602,334 and 4,500,970.
Such automatically guided vehicles are used in a wide variety of applications, including the transfer of raw materials and subcomponent parts in manufacturing and assembly facilities, the cleaning of floors in warehouses and parking lots, and in the delivery of mail in offices. AGV's are also used in a wide variety of applications in the agricultural industry such as, plowing, harvesting, mowing and the like.
In each application, the AGV follows a preselected guide path. Various techniques are used for automatically guiding the vehicle along the preselected guide path. One such technique is disclosed in U.S. Pat. No. 3,039,554. In such a system, the guide path consists of a conductor either laid on the floor or embedded in the floor. An oscillatory signal applied to the conductor produces a magnetic field which is sensed by sensing coils located on the vehicle. Upon detection of the magnetic field, the sensing coils control the steering mechanism of the vehicle to guide the vehicle along the guide path. In U.S. Pat. No 4,003,445, the guide path consists of a fluorescent material, applied t a floor, carpet, or the like which emits visible light in a predetermined frequency range, but is normally invisible under ambient lighting conditions. In this type of guidance system, an ultraviolet light located on the vehicle irradiates the guide path. This radiation stimulates the fluorescent materials in the guide line to emit visible radiation for sensing by a sensor located on the vehicle for controlling the vehicle as it moves along the guide path.
Other systems for guiding an unmanned vehicle along a predetermined path are disclosed in U.S. Pat. Nos. 4,328,545; 4,345,662; 3,933,099; 4,379,497 and 4,500,970. These patents disclose various other optical, radio frequency and ultrasonic techniques for guiding an unmanned vehicle along a preselected guide path.
In addition to providing an automatic guidance system, it is also necessary to provide a collision avoidance system for an unmanned vehicle. Such a system is required to prevent the vehicle from colliding with various stationary and transient obstacles, such as pedestrians, in the path of the vehicle. Both tactile and non-contact collision avoidance systems for automatically guided vehicles are known in the art. For example, U.S. Pat. No. 4,328,545 discloses a tactile system wherein the vehicle is equipped with a front and rear bumper. In such a system, each bumper contains an interlock switch that stops the vehicle upon impact by either bumper with an obstacle.
Tactile sensors alone are unsuitable for applications where a possibility exists of a collision with a pedestrian. This is generally true of all automatically guided vehicles used in an office environment, such as automatically guided mail carts. In such applications, so-called non-contact collision avoidance systems are used. These systems utilize various ultrasonic, radar, infrared and radio frequency apparatus to detect obstacles in the path of the vehicle without contact. For example, U.S. Pat. No. 4,345,662 discloses a non-contact collision avoidance system consisting of a single ultrasonic transceiver mounted on the front of the vehicle for detecting obstacles along a forward path of travel of the vehicle.
An infrared collision avoidance system is disclosed in U.S. Pat. No. 4,627,511. In this type of system, an infrared emitter is affixed to various stationary obstacles along its guide path. Infrared sensors, located on the vehicle, detect the infrared radiation from the emitters affixed to the various obstacles. Once an obstacle is detected, the vehicle is stopped within a predetermined distance from the obstacle.
U.S. Pat. No. 3,039,554 discloses a collision avoidance system for an unmanned vehicle for preventing a collision between two or more automatically guided vehicles which operate along the same guide path. In this system, the guide path comprises an embedded conductor in the floor. The embedded conductor is divided into consecutive sections, each of which is normally deenergized. Once a vehicle is within a particular section of the guide path, control circuitry prevents other vehicles from entering the same section. While such a non-contact avoidance system may prevent collisions between a plurality of automatically guided vehicles along a common guide path, it would not be suitable for preventing collisions between the vehicle and an obstacle, such as a pedestrian, for obvious reasons.
In applications such as, for example, an automatically guided mail cart, it is desirable to provide a non-contact collision avoidance system for all types of obstacles that may be encountered by the vehicle, including pedestrians. Furthermore, in order to keep the cost of the system down, it is also desirable to provide the vehicle with a single collision avoidance system. In this type of application, radar, sonar or infrared detectors are normally used. Conventionally, one or more sensors are located on the front of the vehicle and mounted parallel to the longitudinal axis of the vehicle for detecting obstacles along the forward travel path of the vehicle. An example of such a system is disclosed in U.S. Pat. No. 4,345,662. In this system, the sensor looks ahead to detect obstacles in front of the vehicle. While such a system may be suitable for detecting obstacles in the path of the vehicle while the vehicle is being guided along a straight line path, the system may not perform satisfactorily when the vehicle is turning. During a turn, the sensor may not be "looking" in the same direction as the path of travel of the vehicle. The problem is clearly illustrated in stop action diagram form in FIG. 1. Referring to FIG. 1, the viewing angle of the obstacle sensor in four positions of the vehicle during a turn is shown. As will be noted, for a vehicle with a sensor mounted as shown in FIG. 1, the sensor will not be looking along the path of travel of the vehicle during the turn. As a matter of fact, in such an arrangement, the sensor may detect the wall in position 3 and unnecessarily stop the vehicle before the turn has been completed. Moreover, obstacles in the travel path during a turn may not be detected.
An example of a non-contact collision avoidance system for an automatically guided vehicle which is adapted to detect obstacles around a turn is disclosed in U.S. Pat. No. 4,151,526. In this prior art system, the vehicle is equipped with a radar apparatus for generating a signal which is received by a first antenna located along the guide path. The first antenna is connected via a transmission line to a second antenna also located along the guide path but spaced apart from the first antenna. The second antenna is located around the curve from the first antenna. In this system, electromagnetic waves, transmitted by the radar apparatus located on the vehicle, are received by the first antenna mounted along the guide path. These electromagnetic waves are transmitted from the first antenna to the second antenna by way of an interconnecting transmission line between the two antennas. The electromagnetic wave is then radiated from the second antenna toward the desired obstacle detection zone. If an obstacle is in the zone, the electromagnetic wave will be reflected off the obstacle and reflected back to the second antenna which, in turn, will transmit the electromagnetic wave to the first antenna by way of the interconnecting transmission line. The electromagnetic wave from the first antenna will be received by the radar apparatus located on board the vehicle to indicate the presence of an obstacle in the path of the vehicle around a curve in the guide path. However, even though such a system is able to detect obstacles in the path of the vehicle along a curve in the guide path, such a system requires that antennas be located along the guide path. Consequently, the system disclosed in U.S. Pat. No 4,151,526 would not be practical in certain applications, such as an office environment for automatically guided mail carts.